The disclosed invention relates to methods and apparatus for writing binary information on magnetic media at a predetermined bit density, and more particularly to methods and apparatus for writing the information while the media is moving at a highly variable speed. Suitably, the disclosed invention may be utilized, for example, in a mass transit system for writing information on fare cards. Typically, these fare cards include a magnetic strip on which are recorded information necessary for the operation of the system. Included within this information may be items such as the cost of the ticket, or the area of the city in which the ticket can be used. In operation, the ticket is placed in a ticket reading apparatus by a passenger prior to boarding. The apparatus reads the ticket, determines based on the information contained therein whether or not boarding is to be allowed, and then writes updated information on the ticket.
In the past, tickets were read by transporting them via a servomechanism past read heads and write heads at a predetermined fixed speed. Since the speed was fixed, timing signals for controlling the instants at which the updated information (in the form of magnetic flux changes) were to be written onto the ticket were derived from a fixed frequency oscillator. That is, in the prior art systems, the time interval between the writing of successive bits, as well as the time instant at which magnetic flux changes occurred within a bit to indicate the presence of a 1 or a 0 was fixed in accordance with the predetermined speed at which a servomechanism moved the cards. One problem with these prior art systems is that they require the incorporation of servomechanism circuitry for transporting the cards. It is of course desirable to eliminate this circuitry so as to reduce the cost of the system. To this end, other prior art mass transit systems eliminate the servomechanism by reading the information on the ticket as it is moved by hand past read heads and write heads. But these systems include tachometers to sense the speed of the ticket in order that timing signals can be generated for writing updated information back onto the ticket. A problem, of course, with these tachometers is that besides being a cost factor, they have moving parts which regularly wear out and require maintenance.
Accordingly, it is one object of the invention to provide an improved method for writing information on a magnetic card which is moving at a highly variable speed.
Another object of the invention is to provide improved apparatus for writing information on a magnetic card which is moving at a highly variable speed.
Another object of the invention is to provide apparatus having no moving parts for writing information on a magnetic card which is moving at a highly variable speed.
Still another object of the invention is to provide apparatus for reading magnetically recorded information from one portion of a card which is traveling at a highly variable speed and for simultaneously writing information to a second portion of the card.